This invention relates to an exhaust gas recirculating system for use in an engine of a motor vehicle.
Exhaust gas recirculating systems (this will be referred to as EGR system, hereinafter) for internal combustion engines has found wide use in motor vehicles, because of the application of strict regulation of the public nuisance arising from exhaust gases in an effective attempt to reduce the quantity of harmful nitrogen oxides (this will be referred to as NOx, hereinafter) from the engine. As a result, there have been proposed many systems of this kind. However, those attempts are mostly directed to providing a so-called proportioning type EGR system, in which exhaust gases of a quantity proportional to the quantity of the intake air flow into the engine or the quantity of the exhaust gas flow is recirculated.
However, the proportioning type EGR system suffers from shortcomings in that, since the degree of EGR in the range covering from low load to high load conditions of an engine, i.e., the ratio of the quantity of exhaust gases being recirculated to that of the intake air in the engine is constant, if the degree of the EGR is increased for enhancing the NOx reducing effect in the low load range of an engine, then there results an increase in the ratio of the quantity of inert gases including the residual gases within a cylinder (so-called internal EGR) plus exhaust gases introduced from the exterior of the engine to the quantity of intake air. This in turn leads to instable combustion within an engine cylinder, thus neutralizing the EGR effect.
The mode of an engine for achieving desired EGR characteristic is such that, as best shown by a dotted line in FIG. 10, there is shown a peak in the medium and high load ranges of an engine. The prior art EGR systems are intended to achieve the aforesaid characteristic. However, they fail to achieve the variable EGR characteristic, as shown by the dotted line in FIG. 10, because there is provided only one outlet for recirculating exhaust gases travelling through a flow rate control valve (In general, a single flow rate control valve is provided), which is communicated with the aforesaid exhaust port, into the intake system of an engine.